Magnetic resonance imaging (MRI) systems have been developed for acquiring and reconstructing useful images of subjects such as human patients. In medical diagnostic imaging of the human body, MRI systems are employed in various manners depending upon the particular anatomy, tissue structures, and characteristics to be reproduced in the reconstructed image. Improvements in MRI methodologies and protocols have permitted great improvements in image quality and have overcome such difficulties as long acquisition times during which a patient or tissues within the patient may move, resulting in unwanted image artifacts. One such technique is generally known as Fast Spin Echo (FSE) scanning.
In all MRI systems, a uniform magnetic field, commonly referred to as a polarizing field B.sub.0, is produced. Molecules of a subject positioned within the field attempt to align with the polarizing field and precess at their characteristic frequencies, referred to as Larmor frequencies. A magnetic excitation field B.sub.1 near a Larmor frequency of interest, is then applied to the material to produce a net magnetic moment M.sub.z. This moment may be rotated into a plane transverse to the polarizing field to produce a net transverse magnetic moment M.sub.t. After removal of the excitation field, a magnetic resonance signal is emitted by the material. This signal is then detected and processed to render the desired image.
In addition to the polarizing and excitation fields, gradient fields are typically employed to select a desired slice through the subject and to encode the signals emitted in response to the excitation field. Three orthogonal logical axis are generally defined, including a G.sub.z slice select gradient axis, a G.sub.y phase encoding gradient axes, and a G.sub.x readout gradient axis. The gradient fields produced along the gradient axes result from electrical pulses applied to gradient field coils surrounding the subject. In accordance with various localization techniques, gradients produced on the logical axes result in encoding of emitted signals from the gyromagnetic material within the subject which can be processed, such as through Fourier transformation, to determine the position of individual volume elements, or voxels, within the selected slice of the subject. The signals corresponding to each voxel can then be processed to produce data for corresponding picture elements, or pixels, in a reconstructed digital image.
While rapid signal acquisition sequences are often desirable, such as to avoid image blurring and artifacts caused by movement of the patient, for example, they have been found to result in other types of artifacts. For example, the extremely rapid transitions in gradient fields employed in FSE techniques can give rise to eddy currents producing perturbing magnetic fields. Such eddy currents may be difficult to foresee, and are believed to be linked to such factors as the physical and electromagnetic characteristics of the scanner hardware and support structures. Moreover, because in many scanning sequences, the physical axes of the magnetic fields produced by the gradient coils are rotated to orient the desired slice and to encode tissues within the slice, more complex interplay between eddy currents of the scanner structure may occur which are even more difficult to foresee and compensate.
Techniques have been developed to at least partially measure and compensate for phase errors in FSE imaging sequences (as, for example, from eddy currents). In one known approach, described in U.S. Pat. No. 5,378,985, issued on Jan. 3, 1995 to Hinks, a prescan is performed to determine appropriate compensation levels for certain parameters of a fast spin echo imaging pulse sequence. While such techniques improve image quality significantly, there is still a need for further improvement in FSE imaging sequences, particularly for phase errors that occur in the phase encoding direction .